Power Boats Choosing the Propeller

8/19/2019 Power Boats Choosing the Propeller

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Propellers come in various shapes and sizes, as they have to match the enginepower and RPMs, the boat displacement and the main type of usage expected (ittaes a different propeller to pull a sier fast out of the water, than the one thatshould efficiently and economically push heavy load!"

#: Propeller diameterPitch: #istance the propeller travels in a single turn (theoretical, as it ignores slip!

Engines are usually e$uipped with the manufacturer%s selected propellergenerally suitable for a wide range of boats" So, you got your engine, togetherwith a propeller, and it all runs more or less fine, or so it seems"

&et, taing that the engine power is ade$uate for your boat, you mightoccasionally get that $uestion in the bac of your mind: 'm really using the bestpropeller for my boat) *ould somehow s$ueeze a few nots more, or get the

boat to a faster start) *ould achieve a better cruising economy with a differentpropeller)

+r, your propeller blades have been niced orand eroded after years of use, ordamaged by hitting a submerged log or running aground, and you areconsidering a replacement"

Matching Matching Matching Matching Propeller Propeller Propeller Propeller to to to to Your Your Your Your Boat Boat Boat Boat

' good match between your boat, the engine and the propeller is essential" t isa complex relation, with many inter-related factors" +nly the basic parameters

and very simplified practical methods of selecting the propeller size are givenhere to help you understand the sub.ect better" t is always wise to discuss yourfinal choice with an experienced professional before buying a new propeller"

/he propeller is determined by the number of blades, its diameter and its pitch(definitions shown in the above drawings!, and the direction of rotation (left orright!" /he 0-blade propeller is most often used, but the same definitions arevalid for the 1-blade, 2-blade and multi-blade propellers" /ypically the propeller



size is shown as D x P, where P stands for pitch" /ae as an example a 0-blade304 x 354 propeller: /he diameter is 304, and, you guessed it, the pitch is 354"Sometimes you will see the pitch listed as 35P (that should be read as 435 inchpitch4!"

/he diameter is twice the distance measured from the tip of any blade to thecenter of the propeller hub" 6enerally, propellers with a bigger diameter aremore effective than the smaller ones, especially with heavy displacement boats,as it is more effective to push slower a wider column of water, then to push fasta narrow one" 7ut, with the high speed planing hulls this principle is reversed"

/he $uoted pitch is actually a theoretical value" 7y definition, a propeller with a354 pitch should move 354 in a single revolution" n reality, the actual onerevolution movement is some 38-98 shorter" /his effect is called slip" /heactual amount of slip depends primarily on the boat displacement and inertia(resistance to changing speed!, the engine power, the current RPM

(Revolutions-Per-Minute, the shape of the blades, the friction between the hulland the water, the presence of cavitation andor ventilation, and a number of other factors"

' rotating propeller generates load (resistance! that has to match the enginetor$ue" ;sing a propeller that is to small for the engine will not put the re$uiredload on the driving shaft" 's a conse$uence, the engine may be easily driven intohigh RPMs beyond the recommended maximum, resulting in a permanentdamage" +n the other hand, a too big propeller would overload the engine, notallowing it to achieve its maximum power and optimal woring regime"

/he shape of the propeller blades greatly influences its performance" <igh speedengines are often fitted with propellers that have concave face blades, the socalled cupped blades" /he cupped shape gives a better grip on the water anddelays or reduces cavitation at high speeds" /he improved grip reduces the slipand increases the propeller effective pitch (as a result, propellers with cuppedblades come with pitch reduced by 3 inch!" /he curvature of the blade alsoincreases its strength, so a cupped blade can be made thinner, which additionallyincreases its efficiency at high speeds" 't speeds over 09 nots (below 08 notsthe cupped shape has no effect! the cupped blade effectively increases the speedby some =-31"

TheTheTheThe OptimalOptimalOptimalOptimal PropellerPropellerPropellerPropeller SizeSizeSizeSize

/he propeller blades are set at an angle to its hub" 's the propeller turns, thehydrofoil (same as aerofoil, only in water! shape of the blade generates highpressure on the bac face, and a vacuum on the front" /he difference in the

pressure generates the force that pushes the blade, the propeller and the wholeboat forward"



Cavitation occurs at high speeds (08-09 n, depending on the shape and size of the propeller, the displacement and the design of the hull bottom! as the vacuumon the front face becomes so high that it starts forming imploding bubbles of low

pressure" +ver time, excessive cavitation can result in pitted blade face andpermanent damage to the propeller"

' cavitating propeller may still generate plenty of trust" n contrast, ventilationcauses sharp increase in slip, loss of trust at a sudden burst of the engine RPM"

/he propeller size DxP has to match the tor$ue generated by the engine" /hereare some rough guides in determining the optimal size:

Type of Boat Speed (kn) Slip

'uxiliary sailboats, barges under 5 29

<eavy powerboats, worboats 5 - 39 1=

>ightweight powerboats, cruisers 39 - 08 12<igh-speed planing boats 08 - 29 18

Planing race boats, vee-bottom 29 - 58 38

<ydroplanes, catamarans over 58 ?

• /he slip can never be entirely avoided, but it should be ept as low aspossible" 6enerally it varies between 18 and 98" +n the initial start up,while the propeller has to overcome the boat%s inertia, the degree of slip willbe very high" 's the boat starts moving, the slip lessens" Pushing ahead in ahead sea, the slip will be much higher then in calm water" n extreme

conditions it may approach 388, when the boat is hardly moving ahead"/here is no precise way to determine the actual slip other than to put theselected propeller on and do some test runs under strictly controlledconditions" /he table below lists typical slip values as dependent on the typeof boat and its speed:

• /he same propeller can%t deliver both high speed and maximum power" ' propeller sized for high speed has a small diameter and maximum pitch" ncontrast, propeller that has high trust has a large diameter and lower pitch"7oth diameter and pitch absorb the tor$ue generated by the engine" 's a

rough guide, 3 inch increase in diameter re$uires decrease of 1 - 0 inches inthe pitch for the same load"

• /he higher the pitch that the engine can turn near top horsepower and RPMwithout excessive slip, the faster will the boat go" 7ut there is a practicallimit how far this can go" ncreased pitch induces higher slip, and at somepoint the angle of attac of the propeller blades becomes so high, that the



propeller stalls loosing all trust, much lie an aeroplane that climbs toosteep"

• 't a constant diameter, insufficient pitch can damage the engine by letting itrace at RPMs beyond its design limit" f your engine easily reaches the top

rated RPMs with more spare power, you should change the propeller with alarger size one" @ever run your engine at RPMs beyond its design limit"

• 's a rule of thumb, every 1 inch increase in pitch will decrease the enginespeed by 298 RPM"

• /he combination of the propeller diameter and pitch, and the reduction gearshould be selected to allow the engine to operate at its full rated RPM underfull load"

• ;nlie a car, which has a gearbox to match the engine to different loads, theboat%s reduction gear and propeller are fixed (unless variable-pitch propeller

is used, which we will discuss in another material!" /hat inevitably meansthe engine and the propeller will operate at the optimum level only at oneparticular set of design circumstances, determined primarily by the intendedtype of use of the boat (sport-fishing, racing, water-siing, passenger andcargo transport, heavy load!, while in all other conditions the performancewill be less than optimal"

/he diagram to the right shows a simple method of determining the propellerdiameter that matches the engine 7<P (brae horsepower! and the gearboxreduction"



/ae the example of the Platypus &amaha AourStroe 388 <P outboard enginewith a built-in gearbox with a 1"03 reduction" /he engine produces its 388 7<P at9988 RPM" 't this maximum engine power RPM, the propeller shaft rotates at10B3 RPM (99881"03, rounded!" /he engine design speed limit is =888 RPM,and the shaft turns at 195? RPM (=8881"03!" Aollowing the rule that the

propeller has to generate full load at the engine top RPM, a vertical line startingfrom the 1=88 propeller RPM crosses the 388 7<P 4diagonal4 at roughly 324diameter" 's the actual propeller fitted by &amaha is 304x354, obviously themanufacturer has opted for 34 smaller diameter and a larger pitch, consideringthat the engine will be used mostly for medium to high speed boats where speedis more important than economically pushing heavy load"

Cell, having the propeller design diameter is fine, but we also need to determinethe pitch" f it was not for that always present slip, that would have been easy"Dust tae the maximum design speed of the boat (in nots!, the maximum engineRPMmax (in revolutionsmin! and the nown reduction ratio:

pitch (inches! 3 propeller revolution travel (inches!

speed (nots! speed (nautical mileshour! speed=8 (nautical milesmin! speed F 3B91 =8 (mmin!

speed F 3B91 F 05"0?88B =8 (inchesmin! 3139"110 Fspeed (inchesmin!

n one minute the boat would travel (at no slip!:

pitch (inches! F RPMmax (revmin! F 3 min reductionRatio 3139"110 F speed(inchesmin! F 3 min

from where we calculate the result as:

pitch (inches! 3139"110 F speed F reductionRatio RPMmax

where the value of speed is in nots"

/ae the example of the same 388 <P &amaha Aour Stroe engine mentionedabove" ts RPMmax is =888, and the reduction ratio is 1"03" >et us say we wish toachieve the design speed of 28 nots (theoretical, at no slip!" /hen, we can

calculate the propeller pitch as:

pitch (inches! 3139"110 F 28 F 1"03 =888 3B"?3222 inches G 35 inches(rounded!



Recommended Pitch Ratio (pitchdiameter)

Type of Boat Pitch Ratio Ran!e

Hery heavy cruisers 8"99 - 8"B

'verage cruisers 8"=9 - 3"8

Medium and fast cruisers 8"B - 3"1

<igh-speed cruisers and runabouts 8"5 - 3"9

@ow, this is an excellent result" f we introduce a 18 slip as an average forhigh-speed planing boats (see the table above!, that would mean our boat wouldlose about 18 in speed" So, instead of the theoretical 28 n, it would run atabout 01 n" 6ood to now that, isn%t it)

/here is one more rule of thumb that relates the propeller pitch to the diameter

depending on the type of boat, the pitch ratio (pitchdiameter!, as shown in thetable to the right"

f you review our example, we initially dimensioned the propeller for this 388 <P &amaha as 324 x 354 (# x P!" &amaha has actually used a 304 x 354 stoc propeller" Ce are pretty close, and we could be satisfied with our result"*onsidering that &amaha has a great experience, they now why they went for a304 diameter" /his is a good example why it is always good to consult theprofessionals"

Power Boats Choosing the Propeller

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